The invention relates to a measuring instrument for the depth of the periodontal cavity, comprising a probe needle, a needle holder and a handle, wherein the needle with the needle holder is restrictedly movable from a start position determined by an abutment against the action of a bias spring, the deformation of which caused by the needle movement causes no substantial change of the spring force.
For measuring the periodontal cavity the dentists generally use a measuring instrument consisting of a handle with a holder for a probe needle fixedly attached thereto. It will be clear that the pocket depth thus measured is highly dependent on the probe force applied by the user of the instrument and, therefore, the measuring accuracy at such rigid instruments is low just as the reproduceability of the measurements.
Earlier proposals for a measuring instrument for the depth of the periodontal cavity with a constant adjustable probe force relate to extensive and/or voluminous constructions, such as electronic pressure registration apparatus and complex spring constructions. A measuring instrument for the depth of the periodontal cavity of the last-mentioned type comprises a handle to which a frame is attached, in which frame two shafts extend transverse to the center line of the handle. A bushing is mounted on each of the shafts and a long leaf spring is wound on the two bushings in mutually opposite directions over the respective bushings. This construction is known as a spring motor of the Tensator type, wherein by rotation of one of the bushings the spring wound thereon is unwound and is wound on the other bushing or vice versa, and wherein a constant torque is transferred. In this manner a bias force can be applied in such a manner that as soon as, during the use of the instrument, a force with a value in the order of magnitude of 15 g (0.15 N) is applied at the needle tip, the needle holder borne on one of the two shafts in the frame makes a pivotal movement during which the force opposing this movement remains substantially constant. Even if the user of the instrument would keep moving the handle downwardly after reaching this adjusted force, this movement only results in a relative pivotal movement between the needle holder and the handle without substantially changing the force applied on the bottom of the periodontal cavity by the probe needle. In practice, a pivotal movement of the needle holder with respect to the center line of the handle in the order of 45.degree. is sufficient.
Although the allowable probe needle pressure can thus be accurately maintained very constant, the construction with the Tensator spring is, however, complex, expensive and voluminous, which obstructs the view which should be seen as a disadvantage in view of the place at which the measuring instrument is used. Further, after contamination of the wound springs good operation of the measuring instrument cannot be guaranteed and sterilizing such a Tensator spring construction is not simple. As already mentioned, the chosen spring construction is derived from the Tensator motor which is designed for transferring a constant torque over many shaft rotations and, therefore, unnecessarily complicated for a measuring instrument of the present kind wherein an angle rotation of one axis over a maximum of 60.degree. is necessary.